upgrade from elixir 3s to XTs, what about the fork mount?

Recently picked up some 2012 Shimano XTs to replace my stock Avid Elixir 3s, but haven't had a chance to install them yet. Out of the box, they look boss.

Before I mount them, should I be concerned about the caliper mount for the fork? I'm using (roughly) the same size rotor (went from 185mm cleansweep G2s to 180mm ice tech), so in theory, I could use the Avid mount for the XT caliper, right?

Or are the designs of the caliper different? Holes should line up, right?

Went to several LBS in town, and no one had that Shimano mount in stock, let alone a clear answer.

The difference of 2.5mm (difference in diameter is 5mm, hence difference in radius is 2.5mm) might cause some brake track issues. If you use the 185mm adapters, your caliper will end up being 2.5mm too far from the brake track on the 180mm rotor. The brakes will probably still work, but part of the brake pad will never be in contact with the rotor and a ridge might eventually develop.

the avid rotors have a larger track than the xt brakes, so theres room for play. I used avid rotors with avid adaptors, worked fine.

both the above are true. there is a 99% chance that performance will be exactly the same despite the small difference in size. but, the adapters are cheap. i've ran the wrong adapters with great results, but only temporarily since you might as well have the right stuff.

the avid rotors have a larger track than the xt brakes, so theres room for play. I used avid rotors with avid adaptors, worked fine.

Even with the larger track of the avid rotor, the actual outer edge of the rotor is going to be 2.5mm further away.....the pads will more then likely be a few mm off the top of the rotor.

And of course using avid rotors with avid adaptors things worked fine - you were using a 185mm adaptor with a 185mm rotor. The OP is talking about using a shimano 180mm rotor with an avid 185mm adaptor.

If you use the smaller rotor with the large adapter a ridge will definitely form on the top of the pads where it does not contact the rotor. Eventually when the pads wear down the ridges will press against each other instead of clamping on the rotor, you will lose braking power.